<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        body {
            display: flex;
            flex-direction: column;

        }

        div {
            border: 1px solid pink;
        }

        .son {
            height: 50px;
        }

        .son2-root {
            display: flex;
            flex-grow: 1;
        }

        .son2-root1 {
            width: 200px;
        }

        .son2-root2 {
            flex-grow: 1;
        }
    </style>
</head>

<body>
    <div class="son">
        上
    </div>
    <div class="son2-root">
        <div class="son2-root1">下左</div>
        <div class="son2-root2">下右</div>
    </div>

</body>

</html>